I've been building shooting and hunting with custom single shots for decades and I have two pieces of advice about mega cartridges and one-shooters.
If it is heavy enough to dampen the recoil, all of the weight is in the barrel and it won't handle worth a damn.
It is not really a great idea to hunt dangerous game with a single shot. It certainly has been done, but these days that probibly means the PH will actually kill the animal, and maybe in our face.
SDH